The present invention relates to a plate-shaped, high power, nuclear fuel element containing low enrichment uranium (5 to 20 percent weight uranium.sup.235 in the uranium component) as the fissionable material.
In order to prepare for the possibility that high enrichment uranium, which is used to produce nuclear fuel elements, will not be available in the future, atomic nuclear reactors must be re-equipped to operate with nuclear fuel elements containing low enrichment uranium. Such reactors, for example, are research reactors (including material testing reactors) whose particular significance is their use as a training system for power plant reactor personnel. To be able to perform such re-equipping sensibly and economically without too much expense, studies and tests have been made as to how to change from nuclear fuel elements with about 90 percent enrichment, i.e. 90% U.sup.235 in the uranium component and about 0.4 to 1.3 g U/cm.sup.3, to fuel elements containing low enrichment uranium, i.e. .ltoreq.20% U.sup.235 in the uranium component, without having to encounter too much of a reduction in power during operation of the reactor. Retaining the reactor power seems possible only if the reduction in enrichment from 90% to 20% U.sup.235 is accomplished by correspondingly increasing the uranium density in the fuel material. It has been calculated that for lower power reactors (e.g. between 1 watt and 10 KW.sub.th.), an uranium density in the fuel material up to 2.4 g U/cm.sup.3 would have to be obtained, for medium power reactors an uranium density up to 3.3 gU/cm.sup.3 would have to be obtained, and for high power reactors an uranium density up to 5.75 to 7.03 g U/cm.sup.3, would have to be obtained.
Plate elements have been proposed for re-equipping atomic nuclear reactors with low enrichment uranium.
Uranium-aluminum alloys for fuel elements in plate shape and a process for manufacturing them are disclosed in German Pat. No. 1,118,471. This patent generally relates to the suppression of the formation of UAl.sub.4 in uranium-aluminum alloys. For this purpose, up to 20 atom percent, with respect to the finished product, of an element from the group including Si, Ti, Ge, Zr, Sn, Pb, In, Tl, Fe, Nb and Ga are used as an additional component. The presence of more than 0.5 atom percent of any one of the above-mentioned ternary additional elements results in a UAl.sub.3 concentration of more than 20 percent by weight and a UAl.sub.4 concentration of less than 42 percent by weight. If the proportion of the additional elements is increased, the UAl.sub.4 content is reduced. With the presence of more than 1.2 atom percent of a preferred additional element, an alloy is obtained whose UAl.sub.3 concentration is more than 60 percent by weight and whose UAl.sub.4 content is less than 8 percent by weight. The presence of 5 atom percent and more of the preferred silicon as an additional element leads to the complete suppression of UAl.sub.4 and to an UAl.sub.3 concentration of 65 percent by weight.
In the past, the most complete suppression as possible of UAl.sub.4 formation in an uranium aluminide nuclear fuel element has been desired because the properties of the UAl.sub.4 posed grave problems in the further processing of the nuclear fuel. For example, UAl.sub.4 is hard and brittle, exhibits an orthorhombic lattice/and cannot be rolled into plates. If, as is customary and also disclosed in German Pat. No. 1,118,471, the uranium aluminide fuel material is produced by melting the uranium and aluminum components and pouring the melt into a mold to produce a cast shape or block, without using any additional (suppressing) elements, so much UAl.sub.4 is formed during the subsequent hot rolling process that cracks appear in the fuel material. A uniform and homogeneous distribution of the uranium in the fuel materials, however, is one of the prerequisites for a properly operating nuclear fuel element.
Thus, it is impossible to initially produce UAl.sub.4 by a melting or power technology and to then process it into nuclear fuel plates by means of the well-known picture frame technique (ref. Metals and Fuels, Vol. 1, 1956, pp. 535 to 543). UAl.sub.4 -preproduced either by melting or powder technology - can only be handled by picture frame technique in a mixture with Al powder up to an UAl.sub.4 content of 20.sup.v /o (due to rolling problems). But this concentration does not satisfy the nowadays demands (.gtoreq.2.4 g U/cm.sup.3 as discussed above see page 2, last line).